Currently, due to circumstances of environment, energy problems, and the depletion of resources, an active use of natural energies such as solar energy, wind power, and geothermal heat which are not accompanied by the generation of greenhouse gases without depending on fossil fuels is desired. While solar power generation, wind power generation, and the like in which environmental load is low are spread, effective use of heat energy has been attracting attention. In reality, the amount of the heat energy discharged from trash burning sites, subways, or substations which closely exist is enormous. The temperature of the waste heat discharged from trash burning sites or the like is as high as 300° C. to 600° C., and the temperature of the waste heat in subways or substations is as low as 40° C. to 80° C. Close electric appliances such as computers generate waste heat which is comparatively low (lower than 200° C.) and a lot, and waste heat is generated from various things. The total amount of the energy of the waste heat is great, but the effective energy recollection technology has not been established. As a method of using the energy of the waste heat, there is a thermoelectric conversion element which has been known since a long time ago. In the thermoelectric conversion, since electricity is directly generated from a temperature difference without a driving portion, the loss is lower than in a method of generating electricity by generating vapor by fire power or atomic power and driving a turbine. In addition, no waste matters are generated, and thus the thermoelectric conversion is environmentally friendly. In addition, if a voltage is applied to the both ends of the thermoelectric conversion element, a temperature difference is generated. The Seebeck effect of this thermoelectric conversion was discovered in 1821, but there has been a problem in that the conversion efficiency is low. Recently, as a thermoelectric conversion material having comparatively good efficiency at the temperature of 200° C. or lower, Bi2Te3 is practically applied.
In addition, a thermoelectric conversion material having comparatively good efficiency at approximate room temperature such as Bi—Te can be used as a Peltier device and a cooling element, and can be used in a cooling apparatus that does not use a coolant and has a low environmental load. Recently, according to the continuous increase of the speed of a computer, calories of the heat generated in a circuit in the computer continuously increase. In many circuits and application examples, if heat increases, the performance of a computer decreases. The circuit has to be cooled down so as to be effectively operated. In addition to the circuit in the computer, power devices such as an inverter need the cooling of other elements in some cases. In addition, if the heat can be locally and easily controlled and heat control such as cooling becomes possible, a circuit element in which an element that operates only at a low temperature and an element that generates heat are used in a mixed manner can be applied without a great-scale cooling device. In the cooling of the semiconductor device or the like, it is considered that a thermoelectric conversion material system that conforms well with a silicon is advantageous.
As described above, in viewpoints of both the reuse of waste heat and the cooling of a device, a high performance thermoelectric conversion element is required.
Performance of the thermoelectric conversion material is evaluated by a dimensionless performance index (ZT).
[Expression 1]
                    Z        =                              σ            ⁢                                                  ⁢                          S              2                                κ                                    (        1        )            
Here, σ is an electrical conductivity, S is a Seebeck coefficient, and K is a thermal conductivity, and a value obtained by applying a temperature T on both sides of Expression (1) is ZT. Generally, the higher ZT, the better the performance. Therefore, in Expression (1), it is found that a material in which the Seebeck coefficient S and the electrical conductivity σ in a numerator are high, and the thermal conductivity κ in a denominator is low is preferred as the thermoelectric conversion material. In a Bi—Te-based material, the conversion efficiency is as high as a performance index ZT>1, but Bi and Te are both expensive, and Te is extremely toxic. Therefore, for mass production, cost reduction, and environmental load reduction, a highly effective thermoelectric conversion material to substitute for Bi2Te3 is required.
As a material system having a low environmental load, a thermoelectric conversion material using basically a silicide semiconductor or the full Heusler alloy Fe2VAl is reported in Patent Documents 1 and 2. The silicide semiconductor is a material obtained by a compound of silicon and metal being a semiconductor, and can be configured with a very inexpensive material system. Mg2Si is based on inexpensive Mg and Si, and thus is configured with an inexpensive and intoxic material system. In addition, the full Heusler alloy is configured with inexpensive elements having low environmental loads such as Fe, V, and Al in the same manner, and thus is a material system that does not use toxic rare metals such as a Bi—Te-based material and is valuable in view of industrial applicability. However, in the temperature range of 200° C. or lower, the thermoelectric conversion characteristic thereof does not reach that of the Bi—Te-based material, and thus further research and development are required in the future.
As described above, the performance of the thermoelectric conversion element greatly changes according to the bulk characteristic of the material, but it is reported that a size effect such as particulation also can greatly improve the performance of the thermoelectric conversion element. Interfaces between particles increase by particulating the material to a nanometer size. If the interfaces between the particles increase, the increase becomes the cause of a scattering phonon and thermal conductivity can be greatly decreased. Generally, the performance index ZT of the thermoelectric conversion element is inversely proportional to thermal conductivity, and thus the decrease of thermal conductivity becomes a design guideline extremely important for the performance improvement of the thermoelectric conversion element.
In addition, as the thermoelectric conversion material having the possibility of a low environmental load and a low cost, a transition metal sulfide such as Fe or Ni is also reported in NPL 1. However, in NPL 1, doping elemental species to transition metal sulfide, dependency of concentration, or the carrier density is not controlled, and thus the selection of more optimum doping elemental species and control of the carrier density are needed in order to express a high thermoelectric conversion characteristic.
As described above, attempts to control the size and the characteristic of the material for the thermoelectric conversion element have been made up until now.